1. Field of the Invention
The present invention generally relates to fuel cells, and more particularly to a fuel cell that is compact and has a solid electrolyte layer having proton conductivity.
2. Description of the Related Art
Functions of portable electronic equipments, such as portable telephone sets, portable information terminal equipments, lap-top computers and digital cameras, have diversified, and the power consumption of such portable electronic equipments has increased considerably due to the diversified functions thereof. For this reason, with respect to batteries that are used in the portable electronic equipments, there are demands to realize a high output density and a high energy density.
The batteries that are presently used in the lap-top computers and the portable information terminal equipments do not always satisfy the user's needs, because the driving time per charge is relatively short and the charging time is relatively long. The batteries that are presently used in the portable electronic equipments are lithium-ion secondary batteries. But since it is expected that the energy density required by the portable electronic equipments will become several times the present energy density that is presently required, there are demands to realize a power supply employing a new concept that may replace the lithium-ion secondary battery.
A fuel cell extracts electrical energy from a fuel by causing the oxygen within the air to make an electrochemical reaction with the fuel. The theoretical energy density of the fuel itself in the fuel cell is several times higher than that of the lithium-ion secondary battery. If it is possible to reduce the size of an electrochemical generator part of the fuel cell compared to that of the fuel so as to cause an efficient reaction, there is a possibility of realizing an energy density that is considerably higher than that of the lithium-ion secondary battery. In view of the above, much attention is drawn to the fuel cell that may become the power supply replacing the lithium-ion secondary battery.
The fuel cell types may be categorized into the alkaline, the phosphoric acid, the molten carbonate, the solid oxide electrolyte and the solid polymer electrolyte, depending on the type of electrolyte that is used. With respect to the fuel cell that is used in the portable electronic equipment, there are demands for the fuel cell to have a compact and light structure, to be easy to handle, to be easy to start and stop, and to be resistant against shock and vibration. The solid polymer electrolyte fuel cell is a total solid type which uses a polymer layer as the electrolyte. The solid polymer electrolyte fuel cell has a simple structure, operates even at a relatively low temperature, and can start and stop at a high speed, thereby making the solid polymer electrolyte fuel cell suited for use in the portable electronic equipment. A Direct Methanol Fuel Cell (DMFC) is used particularly in a compact portable electronic apparatus, because of the high energy density of methanol, the ease with which the energy can be accumulated and the simplicity of the cell structure.
In the DMFC, a solid polymer electrolyte layer having proton conductivity is sandwiched between two electrodes, and a methanol aqueous solution is supplied to a fuel electrode. The electrochemical reaction that occurs at the fuel electrode is generated by carbon dioxide, protons and electrons due to the methanol that reacts with water and is oxidized directly at the electrode, as described hereunder. In other words, CH3OH+H2O→CO2+6H++6e−. The protons passes through the solid polymer electrolyte layer and generates water by combining with oxygen at a catalyst layer of an air electrode. In this state, it is possible to obtain and supply the power from the generated electrons to an external circuit by connecting the fuel electrode and the air electrode to the external circuit. The generated water is ejected outside the fuel cell via the air electrode.
In the so-called liquid supply type DMFC, that supplies the methanol aqueous solution directly to the fuel electrode, the methanol concentration gradually decreases within a liquid fuel storage part as the methanol is consumed by the power generation. The power generation stops when the methanol concentration becomes lower than a predetermined concentration, and the methanol included in the methanol aqueous solution is not used up in its entirety.
In order to eliminate this problem, the so-called vapor supply type DMFC, that vaporizes the methanol aqueous solution and supplies the methanol in the vapor state to the catalyst layer of the fuel electrode, has been proposed in a Japanese Laid-Open Patent Application No. 2002-289224, for example. The vapor supply type DMFC can use up the methanol within the liquid fuel in its entirety, because the evaporation of the methanol continues even when the methanol concentration within the liquid fuel storage part becomes lower than a predetermined concentration. In other words, if the volume of the methanol aqueous solution is the same, the vapor supply type DMFC can generate a large amount of power compared to the liquid supply type DMFC.
In the vapor supply type DMFC, carbon dioxide, water and the like are generated by the reaction at the fuel electrode. The pressure at the fuel electrode end increases due to the generated carbon dioxide, and the carbon dioxide blocks a passage of a fuel supply part to cause a reverse flow of the fuel, to thereby result in a state where no fuel is supplied and the reaction stops. For this reason, the carbon dioxide must be ejected outside the fuel cell. On the other hand, the methanol, that is a fuel component, must be kept within the fuel cell to advance the reaction. Hence, it is necessary to efficiently eject only the carbon dioxide, that is an unwanted component, outside the fuel cell.
FIG. 1A is a plan view showing a conventional fuel cell 100, and FIG. 1B is a cross sectional view of the fuel cell 100 shown in FIG. 1A. The fuel cell 100 shown in FIGS. 1A and 1B, that is proposed in the Japanese Laid-Open Patent Application No. 2002-289224, vaporizes a methanol aqueous solution by a fuel holding layer 101, and supplies a methanol gas generated thereby to a fuel electrode 103 that contacts a solid electrolyte layer 102. The reaction described above progresses at a catalyst layer of the fuel electrode 103, and electrons generated by this reaction are obtained via a terminal 104 that contacts the fuel electrode 103. The terminal 104 is provided with a gas emission outlet 105, and a liquid-gas permeation membrane 106 for separating the liquid and gas. The carbon dioxide gas generated at the fuel electrode 103 is ejected outside the fuel cell 100 by the liquid and gas separation of the liquid-gas permeation membrane 106.
The applicants are also aware of a Japanese Laid-Open Patent Application No. 2001-102069.
The Japanese Laid-Open Patent Application No. 2002-289224 discloses that the liquid-gas permeation membrane 106 passes the carbon dioxide, but does not disclose the methanol gas blocking characteristic of the liquid-gas permeation membrane 106. If the liquid-gas permeation membrane 106 does not block the methanol gas, the methanol gas will leak outside the fuel cell, thereby causing the amount of power that is generated to decrease with respect to the amount of methanol supplied. In addition, since the methanol gas is flammable, an excess leak of the methanol gas outside the fuel cell may cause a fire such as ignition and explosion.